In current cellular system, an array of antennas of a network device (e.g., a base station) is usually arranged horizontally; here, FIG. 1a shows dual-polarized antennas arranged horizontally and FIG. 1b shows linear array antennas arranged horizontally. Beams at a transmitting end of the network device can only be adjusted in the horizontal direction, while the beams have a fixed downtilt in the vertical direction; hence, various beamforming techniques and pre-coding techniques are all based on channel information in the horizontal direction. Actually, a wireless signal is transmitted in a three-dimensional manner in space, and the fixed downtilt cannot achieve an optimum system performance. Beam adjustment in vertical direction is important for reducing inter-cell interference and improving the system performance. As antenna technology gets advanced, each antenna element of an active antenna can be controlled independently. By applying such active antenna in an antenna array, the beams can be adjusted in the vertical direction. Here, FIG. 1c shows dual-polarized antennas arranged horizontally and vertically and FIG. 1d shows linear array antennas arranged horizontally and vertically.
With a three-dimensional antenna array, a signal transmitted from a network device can be beamformed not only in the horizontal direction but also in the vertical direction with respect to the terminal. In order to determine a vertical beamforming vector by the network device such that a beam in the vertical direction can be oriented to the terminal to achieve a largest beamforming gain, the terminal usually needs to feed back channel state information (CSI) in the vertical direction.
In the case that the network device supports full-dimension multi-input-multi-output (FD-MIMO) technique, i.e., supports a CSI beamforming transmitting way, the terminal needs to report CSI corresponding to an optimum CSI feedback configuration and location information of the CSI, such that the network device can current optimum beam information. In the case that the network device does not support the FD-MIMO, the terminal needs to report CSI corresponding to each CSI feedback configuration without any location information of the CSI. Since the terminal is not aware of whether the network device is provided with a number of antennas and whether the network device supports the FD-MIMO, the terminal cannot determine the CSI feedback approach.
In view of the above, it is necessary to provide a CSI feedback method and a terminal to enable the terminal to determine the CSI feedback approach based on received information for CSI feedback and to perform CSI measurement and CSI feedback based on the determined CSI feedback approach.